Most personal computers at present contain a magnetic hard disk drive (HDD) as their primary non-volatile storage medium. Solid-state drive (SSD) storage devices (also referred to as “solid-state disks”), however, are becoming increasingly popular as an alternative to the magnetic hard disk. SSD devices typically use electronic memory, either non-volatile memory, such as flash memory, or, in some cases, battery-backed volatile memory, such as synchronous dynamic random access memory (SDRAM). The SSD is typically more costly (in terms of dollars per gigabyte) than the HDD, but has marked advantages such as faster random read access and lower power consumption and audible noise.
It is also possible to configure a computer with both HDD and SSD. For example, U.S. Pat. No. 5,778,418, whose disclosure is incorporated herein by reference, describes a computer system in which a solid-state flash EEPROM system is combined with a rotating disk drive memory to provide mass program and data storage. A common memory controller directs system-generated memory addresses in a disk format to either the EEPROM system or disk memory in a manner that is transparent to the computer system processor. A particular program or data file may then be stored in the portion of the memory system that is best suited to handle it, and thus may take advantage of the different features and characteristics of the EEPROM and magnetic disk memory.
U.S. Patent Application Publication 2005/0240726, whose disclosure is incorporated herein by reference, describes a synergistic hybrid disk drive (SHDD), in which two or more disk drives, with different performance characteristics, communicate directly with each other. This arrangement is said to supply data to a computer system faster and more reliably than would be expected from either individual disk drive. A “SHDD control function” may include statistics regarding the number of times each data file is retrieved by the host computer without being rewritten and the number of times the data file has been rewritten. Seldom-changed data may be stored on a slower but larger capacity disk drive, while more frequently-changed data are stored on a lower capacity but faster disk drive in the SHDD.
U.S. Patent Application Publication 2007/0083575, whose disclosure is incorporated herein by reference, describes automated techniques for storing data in a heterogeneous storage environment based on storage policies. The stated object of these techniques is to store data in storage locations that enable efficient data access while optimizing the use of available storage resources with minimum human intervention. The storage policy specifies when and how data are to be stored and/or migrated and includes rules and scores based on characteristics of the storage devices and of the data files.